A spin momentum transfer method recently proposed is one of promising data writing methods for an MRAM. According to spin transfer magnetization switching, a spin polarized current as a write current is injected into a magnetic recording layer and thereby a magnetization direction of the magnetic recording layer is switched. In a case of conventionally-known magnetization switching due to application of a current-induced magnetic field, a current required for the magnetization switching increases as a size of a memory cell becomes smaller. In contrast, in the case of the spin transfer magnetization switching, a current required for the magnetization switching decreases as a size of a memory cell becomes smaller. Therefore, the spin momentum transfer method is considered to be a promising method for achieving a high-capacity MRAM.
However, in a case where the spin momentum transfer method is applied to a magnetic tunnel junction (MTJ: Magnetic Tunnel Junction) element, it is necessary to overcome a problem of destruction of a tunnel barrier layer. When the magnetization switching is performed by the spin momentum transfer method, the spin polarized current is so supplied in the magnetic tunnel junction as to pass through the tunnel barrier layer. Here, as the case now stands, the spin polarized current as much as mA is required for the spin transfer magnetization switching. To supply such a large current through the magnetic tunnel junction may cause destruction of the tunnel barrier layer.
Moreover, in the case of the magnetic tunnel junction element of the spin momentum transfer type, both of a write current and a read current flow through the same path. Therefore, the so-called read disturb problem that data writing is performed at a time of data reading is caused. The read disturb problem becomes remarkable when the write current is reduced in order to prevent the above-mentioned destruction of the tunnel barrier layer.
In order to avoid the read disturb problem, it is necessary to reduce the read current. In this case, however, a read output becomes smaller and thus a special read circuit is necessary. As a result, it becomes difficult to achieve a high-speed data reading. Moreover, in a case of SoC embedded, circuit overhead is increased, which interferes with the embedded.
One approach to overcome the above-described problems is a technique that causes magnetization switching by supplying a spin polarized current in an in-plane direction of a magnetic recording layer. Such a technique is disclosed, for example, in Japanese Patent Publication JP-2005-191032A, Japanese Patent Publication JP-2005-123617A and U.S. Pat. No. 6,781,871. The spin polarized current is supplied in the in-plane direction of the magnetic recording layer, which causes a domain wall in the magnetic recording layer to move and thereby the magnetization in the magnetic recording layer is switched. From that viewpoint, this technique is called a “domain wall motion method”. An element based on the domain wall motion method is hereinafter referred to as a “domain wall motion element”. According to the domain wall motion method, it is not necessary to supply the spin polarized current so as to pass through the tunnel barrier layer, and thus the destruction of the tunnel barrier layer can be avoided effectively. Furthermore, since respective paths of the write current and the read current are different from each other, the read disturb problem can be avoided.